Pervaporation Membranes and Methods of Use

ABSTRACT

Pervaporation membranes including poly(acrylamide)-grafted alginate membranes, which may be optimized for the separation of alcohols from water, low viscosity sodium alginate membranes containing PEG and PVA, which may be optimized for the separation of organic acids from water, and copolymeric PAN-grafted PVA membranes, which may be optimized for the separation of DMF from water, and methods of making such membranes. Use of such membranes in pervaporation and pervaporation devices containing such membranes. Use of such membranes alone or in combination with ion-exchange membranes for recovery of organic compounds or for water purification applications such as production of potable water or industrial waste treatment. The membranes of the present invention may be used to remove trace amounts of water from organic compounds.

RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.10/348,269 filed Jan. 21, 2003, the contents of which are herebyincorporated in its entirety by reference.

FIELD OF THE INVENTION

The present invention relates to a process for making pervaporationseparation membranes that can be used for the separation of organicsubstances, particularly volatile organic substances, from water. Moreparticularly, it relates to novel processes for producing and usingmembranes for the effective separation of alcohols, organic acids andother organic compounds from water and vice versa. The inventionadditionally includes pervaporation membranes made by the processes ofthe invention and methods of using such membranes.

BACKGROUND OF THE INVENTION

A number of organic compounds may be found in water, particularly watercontaminated by various industrial processes. It is desirable to removesuch compounds from the water for a large variety of reasons, rangingfrom water purification to recovery of the organic compounds.

As is well known to those skilled in the art, it is possible to removewater from mixtures thereof with organic liquids by various techniquesincluding adsorption or distillation. These conventional processes,particularly distillation, are however, characterized by high capitalcost. In the case of distillation, for example, the process requiresexpensive distillation towers, heaters, heat exchangers (reboilers,condensers, etc), together with a substantial amount of auxiliaryequipment typified by pumps, collection vessels, vacuum generatingequipment, etc. Such operations are characterized by high operatingcosts principally costs of heating and cooling-plus pumping, etc.

Furthermore, the properties of the materials being separated, as isevidenced by the distillation curves, may be such that a large number ofplates may be required, etc. When the material forms an azeotrope withwater, additional problems may be present which for example, wouldrequire that separation be effected in a series of steps (e.g. as in twotowers) or by addition of extraneous materials to the system. There arealso comparable problems which are unique to adsorption systems.

It has been found to be possible to utilize membrane systems to separatemixtures of miscible liquids by pervaporation. In this process, thecharge liquid is brought into contact with a membrane film; and onecomponent of the charge liquid preferentially permeates the membrane.The permeate is then removed as a vapor from the downstream side of thefilm-typically by sweeping with a carrier gas or by reducing thepressure below the saturated vapor pressure of the permeating species.

A number of pervaporation membranes have been developed for separationof azeotropes and organic compounds from water. (See Aminabhavi, T. M.;Khinnavar, R. S.; Harogoppad, S. B.; Aithal, U. S.; Nguyen, Q. T.;Hansen, K. C. J. Macromol Sci.-Rev Macromol Chem Phys 1994, C43, 139;Li, S.; Tuan, V. A.; Noble, R. D.; Falconer, J. L. Ind Eng Chem Res,2001, 40, 4577; Yeom, C. K.; Jegal, J. G.; Lee, K. H. J Appl Polym Sci1996, 62, 1561; Shieh, J. J.; Huang, R. Y. M. J Membrane Sci 1998, 148,243; Okuno, H.; Uragami, T. Polymer 1992, 33, 1459.) Examples of suchmembranes are described in U.S. Pat. No. 2,953,502 issued on Sep. 20,1960 to Binning, R. C. and Lee, R. J and in Neel, J.; Nguyen, Q. T.;Bruschke, H. Europaaiaches Patentamt Anmeidung 90123133.2, Dec. 21,1990. Other pervaporation membranes include blend membranes (Chanachi,A.; Jiraratananon, R.; Uttapap, D.; Moon, G. Y.; Anderson, W. A.; Haung,R. Y. M., J Membr Sci 2000, 16, 6271; Toti, U. S.; Kariduraganavar, M.Y.; Soppimath, K. S.; Aminabhavi, T. M., J Appl Polym Sci 2002, 83,259), composite membranes (Meier-Haack, J.; Lenk, W.; Lehmann, D.;Lunkwitz, K., J Membr Sci 2001, 184, 233; Qureshi, N.; Meagher, M. M.;Huang, J.; Hutkins, R. W., J Membr Sci 2001, 187, 93), charged membranes(Huang, S. C.; Ball, I. J.; Kaner, R. B, Macromolecules 1998, 31, 5456;Kusumocahyo, S. P.; Sudoh, M, J Membr Sci 1999, 161, 77), polyioncomplex membranes (Jegal, J.; Lee, K.-H., J Appl Polym Sci 1996, 60,1177), copolymer membranes (Park, C. H.; Nam, S. Y.; Lee, Y. M.;Kujawski, W., J Membr Sci 2000, 164, 121), and grafted copolymermembranes (Ping, Z. H.; Nguyen, Q. T.; Chen, S. M.; Ding, Y. D., J MembrSci 2002, 195, 23).

Natural polymers have also been used in pervaporation membranes. (SeeOkuno, H.; Uragami, T., Polymer 1992, 33, 1459; Zhang, L.; Zhou, D.;Wang, H.; Cheng, S. J Membr Sci 1997, 124, 195; Bhat, N. V.; Wavhal, D.S., J Appl Polym Sci 2000, 76, 258; Huang, R. Y. M.; Moon, G. Y.; Pal,R. J Membr Sci 2001, 184, 1; Chanachi, A.; Jiraratananon, R.; Uttapap,D.; Moon, G. Y.; Anderson, W. A.; Huang, R. Y. M. J Membr Sci 2000, 166,271; Cao, S.; Shi, Y.; Chen, G. J Membr Sci 2000, 165, 89; Soppimath, K.S.; Aminabhavi, T. M.; Kulkarni, A. R.; Rudzinsiki, W. E. J Control Rel2001, 70, 1; and Jiraratananon, R.; Chanachai, A.; Huang, R. Y. M.;Uttapap D. J Membr Sci 2002, 195, 143.)

Among natural polymer membranes, sodium alginate (SA) membranes areknown to have superior pervaporation separation characteristics whenused to separate mixtures of water and methanol or water and ethanol.(See Uragami, T.; Saito, M. Sep Sci Technol 1989, 24, 541.) Sodiumalginate is a water-soluble polysaccharide that may be gelled by acidtreatment or by crosslinking with glutaraldehyde or Ca²⁺ ions. Sodiumalginate is a copolymer composed of 1→4)-linked β-D-mannuronic acid (M)and α-L-guluronic acid (G) residues arranged in blockwise fashion.

Three different types of blocks are possible: homopolymeric MM blocks,homopolymeric GG blocks and heteropolymeric, sequentially alternating MGblocks. (See Fischer, F. G.; Dorfel, H. Hoppe Seyler's Z. Physiol Chem1955, 302, 186; and Huang, A.; Larsan, B.; Smidsroed, O. Acta Chem Scand1966, 20, 183.) Properties of the polymer vary based one the amount ofα-L-guluronic acid (G). (See Moe, S. T.; Draget, K. I.; Break, G. S.;Smidsrod, O.; Alginates, in A. M. Stephen (Ed.), Food Polysaccharidesand their Applications, First Ed., Marcel Dekker, New York, 1995, pp.245-286.)

Mochizuki et al., studied the relationship between permselectivity ofalginic acid membrane and its solid state structure as well as theeffect of counter cations on membrane performance (Mochizuki, A.; Amiya,S.; Sato, Y.; Ogawara, H.; Yamashita, S. J Appl Polym Sci 1990, 40,385). Aminabhavi et al. have prepared blend membranes of SA withpolyacrylamide-grafted-gaur gum and studied their PV separationcharacteristics for acetic acid+water and isopropanol+water mixtures(Toti, U. S.; Kariduraganavar, M. Y.; Soppimath, K. S.; Aminabhavi, T.M. J Appl Polym Sci 2002, 83, 259; Toti, U. S.; Aminabhavi, T. M. J ApplPolym Sci 2002, Accepted). Additionally, some studies have also beencarried out to understand the effect of polymer viscosity on diffusionof drugs using calcium alginate membrane coated tablets. (See Bhagat, R.H.; Mendes, R. W.; Mathiowotz, E.; Bhargava, H. N. Drug Dev Ind Pharm1994, 20, 387; Kikuchi, A.; Kawabuchi, M.; Watanabe, A.; Sugihara, M.;Sakurai, Y.; Okano, T., J Control Rel 1999, 58, 21.)

However, sodium alginate membranes suffer from lack of mechanicalstability. This problem can be corrected somewhat by cross-linking themembranes (Yeom, C. K.; Lee, K. H., J Appl Polym Sci 1998, 67, 209),blending the sodium alginate with other stable polymers (Yeom, C. K.;Lee, K. H., J Appl Polym Sci 1998, 67, 949), or by developing compositemembranes (Huang, R.Y. M.; Pal, R.; Moon, G. Y. J Membr Sci. 2000, 166,275; Kurkuri, M. D.; Toti, U. S.; Aminabhavi, T. M. J Appl Polym Sci2002, Accepted; Yang, G.; Zhang, L.; Peng, T.; Zhong, W. J Membr Sci2000, 175, 53). Several asymmetric membranes have been prepared as thinfilm composites of SA with different hydrophilic and hydrophobic supportmaterials. (See Moon, G. Y.; Pal, R.; Huang, R. Y. M. J Membr Sci 1999,156, 17-27; Huang, R. Y. M.; Pal, R.; Moon, G. Y. J Membr Sci 2000, 166,275; and Wang, X. N. J Membr Sci 2000, 170, 71.) However, even sodiumalginate membranes with these improvements in mechanical stabilityremain unsuitable for many uses.

In addition to addressing mechanical stability problems, achieving thesimultaneous enhancement of both selectivity and flux or enhancement ofone characteristic without decrease of the other is a challenging taskin the area of pervaporation membranes. To achieve this goal, manyefforts have been made to fabricate or modify different types ofmembranes. (See Huang, R. Y. M.; Pal, R.; Moon, G. Y. J Membrane Sci1999, 160, 17; Jo, W. H.; Kim, H. J.; Kang, Y. S. J Appl Polym Sci 1994,51, 529; Kim, J. H.; Lee, K. H.; Kim, S. Y. J Membrane Sci 2000, 169,81; and Lee, K. R.; Teng, M. Y.; Lee, H. H.; Lai, J, Y. J Membrane Sci2000, 164, 13.) For instance, efforts from different groups haveutilized different types of membranes for the pervaporation separationof aqueous-organic mixtures. (See Kurkuri, M. D.; Kumbar, S. G.;Aminabhavi, T. M. J Appl Polym Sci 2002, In press; Kurkuri, M. D.; Toti,U. S.; Aminabhavi, T. M. J Appl Polym Sci 2002, In press; Toti, U. S.;Kariduraganavar, M. Y.; Soppimath, K. S.; Aminabhavi, T. M. J Appl PolymSci 2002, 83, 259; Aminabhavi, T. M.; Naik, H. G. J Appl Polym Sci 2002,83, 244; and Aminabhavi, T. M.; Naik, H. G. J Appl Polym Sci 2002, 83,273.) However, improvements in flux or selectivity remain useful forimproving overall pervaporation membrane quality and for allowingadditional uses of such membranes.

SUMMARY OF THE INVENTION

The present invention includes an acrylamide grafted alginate membrane.In one embodiment, the membrane has an acrylamide monomer to alginatepolymer ratio of between about 1:2 and 1:1 and is cross-linked withglutaraldehyde. The alginate may be sodium alginate or any other type ofalginate. Other cellulosic polymers may also be used. The membrane maybe optimized for the separation of alcohol, such as isopropanol, andwater. PEG or PVA may also be included in the membrane.

The invention additionally includes a method of forming a pervaporationmembrane by first mixing alginate with acrylamide monomer in a ratio ofbetween 1:2 and 1:1. In one embodiment, sodium alginate may be used,although use of other alginate sources or other cellulosic polymers maybe acceptable. Potassium persulfate, for example in an amount of around3 g per every 1 g of sodium alginate polymer, may them be added to themixture and the resulting polymer precipitated. The polymer may then bemixed with water and PEG and PVA added to the mixture. In certainembodiments, approximately 10 mass % PEG and approximately 5 mass % PVAare added. Finally, this mixture may be cast as a membrane andcross-linked with glutaraldehyde. In certain embodiments, the membraneis cross-linked in a solution of water and alcohol in a ratio ofapproximately 25:75 additionally containing approximately 1 volume %glutaraldehyde. The membrane may be optimized for the separation ofalcohol, such as isopropanol, methanol or ethanol, from water.

The invention also includes a pervaporation membrane including lowviscosity sodium alginate, PEG and PVA and cross-linked withglutaraldehyde. In certain embodiments, the membrane includesapproximately 10 mass % PEG and between approximately 5 and 20 mass %PVA. Specific embodiments include between approximately 10 mass % PEGand approximately 5 mass % PVA. The membrane may be optimized for theseparation of organic acids, such as acetic acid, from water.

A novel method of the invention relates to forming a pervaporationmembrane by first forming a solution of low viscosity sodium alginatepolymer, for example polymer with a viscosity of approximately 138 mPa.sfor a 5% solution, and adding PEG and PVA to the solution. In certainembodiments approximately 10 mass % PEG and between 5 and 20 mass % PVAare added to the solution. A membrane may be cast from the polymersolution and then cross-linked with glutaraldehyde, for instance asolution of water and alcohol in a ratio of 25:75 containingapproximately 1 volume % glutaraldehyde. Such membranes may be optimizedfor the separation of organic acids such as acetic acid from water.

Another aspect of the invention relates to the preparation of acopolymeric PAN-grafted PVA membrane. In certain embodiments, themembrane has grafting ratios between 46% and 93%. Increasing the % PANin such membranes may increase their selectivity. Certain membranes maybe optimized for separation of DMF from water.

Another method of the invention relates to forming a pervaporationmembrane by forming a solution of PVA in DMSO under a nitrogenatmosphere, adding PAN to the solution, in certain embodiments in arange of 46% to 93% grafting calculated considering mass, then addingCAN to the solution and precipitating the polymer. Another solution maythen be prepared from the polymer and cross-linked, for example withglutarahdehyde, then cast as a membrane. Such membranes may be optimizedfor the separation of DMF from water.

The invention also includes a method of separating organic compoundsfrom water using any of the above membranes. The membrane may be placedbetween two chambers in a pervaporation device with at least twochambers. Water mixed with the organic compound may be supplied to onechamber and pressure decreased in the second chamber on the other sideof the pervaporation membrane. For instance, the method may be used toseparate organic acids such as acetic acid, alcohols such asisopropanol, methanol or ethanol, of DMF from water.

Another aspect of the present invention relates to a pervaporationdevice with at least two chambers and any pervaporation membranedescribed above affixed between the two chambers in manner to preventfluid flow from one chamber to the other.

The invention additionally includes water purification systems includinghybrid processes i.e., one ion-exchange unit and at least onepervaporation unit or distillation unit, if necessary. The membrane usedin the pervaporation unit may be selected from any of the membranesdescribed above. Multiple ion exchange units or pervaporation units maybe combined to use hybrid or multiple membranes of each type. In certainembodiments hybrid pervaporation units are included and some unitscontain membranes optimized to separate different organic compounds fromwater than the membranes in other units. In exemplary embodiments, theion exchange membranes are 4-vinyl pyridine anion exchange membranescrosslinked with aniline and epicholorohydrin, cation exchange membranesformed from formaldehyde-crosslined PVA, and/or cation exchangemembranes formed from brominated PVA. These systems may be used forvarious water purification applications, such as production of potablewater and purification of industrial waste, for instance to meetenvironmental control standards. The systems may also form part of arecovery process to remove ions and/or organic compounds from water.Such recovery processes may be coupled to purification process and thesystems may be used with both and additional ends in mind.

For a better understanding of the invention and its advantages,reference may be made to the following description of exemplaryembodiments, taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be further understood through reference to thefollowing drawings in which:

FIG. 1 illustrates a method for preparing a poly(acrylamide)-gratedsodium alginate membrane according to an embodiment of the presentinvention;

FIG. 2 illustrates a method for preparing a low viscosity sodiumalginate membrane containing polyethylene glycol and poly(vinyl) alcoholaccording to an embodiment of the present invention;

FIG. 3 illustrates a method for preparing a polyacrylenitrile-graftedpoly(vinyl) alcohol membrane according to an embodiment of the presentinvention; and

FIG. 4 illustrates a prior art pervaporation process which may be usedwith membranes of the present invention or in hybrid processes of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention includes pervaporation membranes with desirablemechanical, flux, and/or stability characteristics. It also includesprocesses for the production and use of such membranes.

One embodiment of the present invention relates topoly(acrylamide)-grafted-sodium alginate membranes which may be used forthe separation of alcohols from water. Such membranes may be prepared byfree radical polymerization using a sodium alginate polymer toacrylamide monomer ratio of about 2:1 or 1:1. To increase the membranestability, the membranes may be cross-linked with glutaraldehyde. Themembranes of this embodiment are useful in separating organic alcohols,particularly isopropanol from aqueous mixtures. The membranes areefficient for such separations in the temperature range of approximately30-50° C. The membranes are particularly effective in separating 10 to50 mass % and particularly approximately 20 mass %isopropanol-containing aqueous mixtures. Similar mass % aqueous mixturesof other alcohols may also be separated using the membranes of thisembodiment.

Another embodiment of the present invention relates to sodium alginatemembranes prepared with polyethylene glycol (PEG) and poly(vinyl)alcohol (PVA) and with different viscosity grade sodium alginate tooptimise their capacity for the separation of water/acetic acidmixtures. Such membranes also exhibit improved membrane flexibility.

Specifically, the membranes may be prepared using a selected viscositygrade sodium alginate that is cross-linked with glutaraldehyde in a50:50 volume water:ethanol mixture. The membranes may be cast using thesolution casting method known to the art. Specific optimized membranesof this embodiment are able to achieve suitable separation of aceticacid:water mixtures or other organic acid:water mixtures of 10-50 mass %water at approximately 30° C. Membranes prepared from low viscositygrade polymer exhibit the highest separation selectivity for organicacid:water mixtures containing 10 to 20 mass % water. Membranes preparedfrom higher viscosity grade polymers exhibit higher flux, but lowerselectivity. Accordingly, polymer viscosity for membranes of the presentembodiment may be selected to achieve the desired balance betweenselectivity and flux for a given application.

However, flux may also be increased by adding PEG and PVA to membranesprepared with low viscosity grade polymers. Specifically, membranesprepared with approximately 10 mass % PEG and 5 mass % PVA exhibit highflux and selectivity. Membranes prepared with approximately 10 mass %PEG and 10 mass % PVA have lower selectivity than those prepared with 5mass % PVA, but exhibit higher flux. Accordingly, the present inventionalso includes membranes prepared with low viscosity grade sodiumalginate and PEG, to which PVA is added to increase flux. The amount ofPVA in such membranes may be varied to achieve the desired balancebetween selectivity and flux for a given application.

Although the previous embodiments have focused on membranes preparedfrom sodium alginate, it will be apparent to one skilled in the art thatother alginates may be readily substituted for sodium alginate.Additionally, other natural or synthetic polymers with propertiessimilar to sodium alginate may also be used to construct membranes thatwill benefit from the techniques described above. Such membranes arealso within the scope of the present invention.

Another embodiment of the invention relates to copolymericpolyacrylonitrile-grafted-PVA membranes which are useful for theseparation of dimethyl formamide (DMF)and water mixtures. In a specificembodiment, such membranes are modified to obtain an optimum combinationof flux and selectivity. Membranes containing approximately 46% to 93%polyacrylonitrile (PAN) may be used to separate DMF:water mixtures ofbetween approximately 10 to 90 mass % water at temperatures from about25 to 45° C.

All of the membranes described above may be used for water purificationthrough separation of organic compounds. Such organic compounds may benaturally present in the water or may be the result of a non-naturalactivity such as an agricultural or industrial process or pollution. Thepurified water may be purified to drinking quality according tostandards set forth by organizations such as the World HealthOrganization. It may also be purified for industrial uses or prior torelease into the environment. Recovery processes of the presentinvention may also be used to recapture organic material.

Membranes of the present invention may be used to separate alcohols,organic acids, or DMF from water. In particular, thepoly(acrylamide)-grafted sodium alginate membranes and similar membranesmay be used to separate alcohols, such as ethanol, isoproponal, andmethanol from water. The sodium alginate membranes containing PEG andPVA, in particular the low viscosity sodium alginate membranes, are wellsuited for separation or organic acids, such as acetic acid, from water.Finally, the PAN-grafted PVA membranes are useful for separating DMFfrom water. These membranes may be used alone or in combination toremove a variety of organic contaminants.

The membranes of the present invention may be utilized in currentpervaporation systems. Any slight modifications that may be requiredwill be apparent to one skilled in the art. The increased mechanicalstrength of the membranes of the present invention may allow for lessfrequent replacement when used with conventional systems. Additionally,the membranes of the present invention may be able to withstand greaterstresses, such as lower pressure on one side of the membrane, than someconventional pervaporation membranes.

In one embodiment, pervaporation purification using membranes of thepresent invention may be coupled with ion exchange purification. Variousion exchange membranes are known to the art. In a specific embodiment,ion exchange membranes as described in U.S. patent application Ser. No.10/007,442 filed Dec. 5, 2001(the '442 application, incorporated byreference herein) are used. An ion exchange tank as described in the'442 application may be coupled to one or more pervaporation tanks. Suchtanks may be placed before or after the ion exchange tank. More than onetank may be used to contain pervaporation membranes optimized toseparate specific organic molecules.

Although only preferred embodiments of the invention are specificallydescribed above and in the following examples, it will be appreciatedthat modifications and variations of the invention are possible withoutdeparting from the spirit and intended scope of the invention.

EXAMPLES Example 1 Synthesis of poly(acrylamide)-grafted-sodium AlginateCopolymer

Sodium alginate, ammonium persulfate, isopropanol, acetone and methanolused in the following example were all of analytical reagent (AR) grade.Poly(acrylamide)-grafted-sodium alginate copolymer was prepared bypersulfate induced radical polymerization using sodium alginate polymerto acrylamide monomer ratios of 2:1 or 1:1. A 10 mass % aqueous solutionof sodium alginate was prepared in a three-necked round bottom flask andstirred vigorously for 1 h after adding the correct ratio of acrylamidemonomer at 70° C. Then, a 100 mL solution containing potassiumpersulfate initiator at a concentration of 10-3 moles (0.03 g/lg ofexpected polymer) was added and the reaction was continued for 10 h at70° C. under nitrogen atmosphere. Free radical sites were generated byabstracting hydrogen from the —OH group of the polymer to facilitate thegrafting of acrylamide onto sodium alginate. The mass obtained wasprecipitated in acetone and washed with a water:methanol (7:3 by volumemixture to remove the homopolymer formed. The solid polymer was dried inan electrically controlled oven at 40° C., weighed and stored in adesiccator until further use. The monomer conversion was up to 90% forboth the copolymers as indicated in Table I, which also providessynthetic details. TABLE I Synthetic Details ofPolyacrylamide-grafted-Sodium Alginate Copolymers Mass Mass Mass of ofof Initiator Grafting % Sample Polymer AAm (g/g of % Effi- ConversionCode (g) (g) polymer) Grafting ciency of AAm NaAlg 0 0 0 0 0 0 NaAlg-110 5 0.3 44 ± 1 96 ± 1 89 ± 1 NaAlg-2 10 10 0.3 94 ± 1 98 ± 1 95 ± 1The sodium alginate is pure polymer without grafting while NaAlg-1 and-2 are the grafted copolymers of sodium alginate and acrylamide.

The reaction scheme for the graft copolymerization is as follows:

Example 2 Polymer Characterization of poly(acrylamide)-grafted-sodiumAlginate

Polymers prepared as described in Example 1 were characterized byFourier transform infrared (FTIR) spectra of the polymer samples in KBrpellets using a Nicolet, Model Impact 410, USA in the wavelength regionof 4000 to 400 cm⁻¹. For the neat NaAlg, a characteristic broad bandappearing at ˜3420 cm⁻¹ corresponds to O—H stretching vibrations ofNaAlg. A sharp peak observed at ˜1616 cm⁻¹ corresponds to carbonyl groupof —COONa moiety present in NaAlg. In the spectra of grafted copolymers,shoulder peaks appearing at ˜3190 cm⁻¹ and a sharp peak at ˜1672 cm⁻¹corresponding to N—H stretching and C═O stretching vibrations,respectively and confirm the grafting reaction. A new peak at ˜1450 cm⁻¹in case of NaAlg-1 and NaAlg-2 corresponds to C—N bending vibrationfurther providing evidence of the grafting reaction.

Viscosity measurements of the aqueous solutions of sodium alginate(NaAlg) and the grafted copolymers (NaAlg-1 and NaAlg-2) were,respectively prepared in mass % concentration of 0.25, 1.0 and 1.0 at30° C. using Schott-Gerate Viscometer (Model AVS 350, Germany). Thekinematic viscosity of 0.25 mass % solution of neat NaAlg is 1.65 Stokeswhile for 1 mass % solution of NaAlg-1 and NaAlg-2 polymers, thesevalues are 2.11 and 2.96 Stokes, respectively. Decrease in viscosity ofNaAlg-1 and NaAlg-2 solutions confirms the grafting reaction. Higherviscosity observed for NaAlg-2 when compared to NaAlg-1 may be due tothe increased chain length and hence the increased entanglement of thepolymer chain.

Example 3 Fabrication of poly(acrylamide)-grafted-sodium AlginateMembranes

The following membranes were used in Examples 4-8. A 5% solution by massof the poly(acrylamide)-grafted-sodium alginate copolymer (as describedin Examples 1 and 2) was prepared in 100 mL distilled water. To this, 25ml of a previously prepared solution of polyethylene glycol (PEG)sufficient to achieve 10 mass % PEG and 25 ml of a previously preparedsolution of poly(vinyl alcohol)(PVA) sufficient to achieve 5 mass % PVAwere added and stirred for 12 h at room temperature The solution wasfiltered through a cotton plug and membranes were cast on a leveledglass plate and dried at room temperature. The membranes obtained werepeeled off from glass plate and cross-linked in acidic (25:75)water:methanol mixture containing 1.0 vol. % of glutaraldehyde for 24 h.The membranes were designated, respectively as NaAlg, NaAlg-1 andNaAlg-2 representing neat sodium alginate membrane, 2:1alginate:acrylamide copolymer membrane and 1:1 alginate:acrylamidemembrane, respectively. Membranes thus prepared were stored in a dryatmosphere at room temperature and used in separation studies.

In another variation of the membrane preparation process, membranesotherwise similar to the NaAlg, NaAlg-1 and NaAlg-2 membranes above werecrosslinked by glutaraldehyde in an acidic water:ethanol (25:75 byvolume) mixture. Ethanol, being a non-solvent, helped to prevent thedissolution of the membrane and the water present in solution may beresponsible for membrane swelling, further facilitating crosslinking inthe presence of glutaraldehyde. Both of the copolymeric membranes werebrittle and this may be due to the crystalline nature of acrylamidepresent. To increase the flexibility of copolymeric membranes, 10 mass %of PEG and 5 mass % of PVA were added to the membranes. Addition of 10mass % of PVA resulted in a phase separation in both the NaAlg-1 andNaAlg-2 types of membranes.

Example 4 Sorption Runs

The tests of the present example provide information regarding membranestability. Dynamic and equilibrium sorption experiments on membraneswere performed in water+isopropanol feed mixtures at 30±0.5° C. in anelectronically controlled oven (WTB Binder, Germany). For these studies,circularly cut (≈4.00 cm diameter) disk shaped membranes were kept in avacuum oven at 25° C. for 48 h before use. Initial mass of thesemembranes was measured on a top loading single pan digital microbalance(Model AE 240, Switzerland) sensitive to ±0.01 mg. Samples were placedinside the air-tight test bottles containing different compositions ofwater+isopropanol mixtures. Test bottles were placed in an ovenmaintained at the constant temperature of 30° C. Mass measurements wereperformed at suitably selected time intervals by removing samples fromthe test bottles, wiping the surface-adhered solvent drops by gentlypressing them in between filter paper wraps, weighing the samplesimmediately, and again placing them back into test bottles in the oven.In order to minimize the errors due to evaporation losses, this step wascompleted within 15-20 s. From gravimetric data, equilibrium mass %uptake (M_(∞) or S) by the membrane was calculated from the initial drymass of the membrane, W₀ and the mass of the membrane, at equilibriumW_(∞) (˜48 h) as, $\begin{matrix}{S = {\frac{W_{\infty} - W_{0}}{W_{0}} \times 100}} & (1)\end{matrix}$

These data are presented in Table II below. TABLE II Equilibrium Mass %Uptake (S) and Sorption Selectivity (α_(sorp)) of the Membranes at 30°C. Mass % Wa- ter in the S (kg/kg) eq. (1) α_(sopt) eq. (2) Feed NaAlgNaAlg-1 NaAlg-2 NaAlg NaAlg-1 NaAlg-2 10 14.43 10.77 15.00 60.2 853325.83 20 21.41 22.41 30.68 21.3 61.7 172.7 30 31.27 33.98 45.30 14.018.6 26.7 40 36.74 46.88 60.81 22.4 13.4 21.5 50 44.83 62.42 78.17 9.510.7 16.9 60 51.29 71.80 90.29 6.0 10.4 5.2 70 60.83 89.31 131.9 4.9 7.35.7 80 66.75 104.4 151.9 4.7 6.0 6.7For calculation of selectivity see Example 5.

Example 5 Sorption Selectivity (α_(sorp))

Each completely equilibrated membrane in each different mass % watercontaining mixture from Example 4 was removed from the test bottle andthe surface adhered liquid drops were removed by blotting with a tissuepaper. It was then placed inside the glass-trap connected to anothercold trap surrounded by liquid nitrogen and then heated to 110° C.(beyond the boiling temperature of water). The vapor was condensed inthe cold trap surrounded by liquid nitrogen jar. Composition of thecondensed liquid mixture was then determined by measuring its refractiveindex (accurate to ±0.0002 units) using an Abbe Refractometer (Atago 3T,Japan) and by comparing it with a standard graph. Sorption selectivitywas calculated using, $\begin{matrix}{\alpha_{sorp} = \frac{M_{W}/M_{{Iso} - {OH}}}{F_{W}/F_{{Iso} - {OH}}}} & (2)\end{matrix}$where M_(W), M_(Iso-OH) and F_(W), F_(Iso-OH) are the mass % of waterand isopropanol, respectively in the membrane and the feed mixture. Theresults of sorption selectivity are included in Table II.

Equilibrium mass % uptake of water-isopropanol mixture increased with %grafting of the membrane (see Table II) at all the compositions ofwater-isopropanol mixture except at 10 mass % of water in the feedmixture. The results of asorp are higher at 10 mass % of water in thefeed mixture and drastically decrease with an increasing amount of waterin the feed mixture. Also, the α_(sorp) values increase with increasinggrafting i.e., from the neat NaAlg to the grafted copolymer membranesfor all the concentrations of water in the feed mixture.

Example 6 Pervaporation Tests

Pervaporation (PV) experiments were performed using the module describedin Aminabhavi, T. M.; Naik, H. G. J Appl Polym Sci 2002, 83, 244.Effective surface area of the membrane in contact with the feed mixturewas 32.4 cm² and the capacity of PV cell used was about 250 cm³. Themass % of water in feed mixtures with isopropanol was varied from 10 to80. After placing 150 mL of the feed mixture in the feed compartment foreach test, the test membrane was allowed to equilibrate for 2 h. Then,the downstream side of the PV apparatus was continuously evacuated byusing a vacuum pump (Toshniwal, India) at a vacuum pressure of 10 torrand permeate mixture was condensed in liquid nitrogen traps. Mass of thepermeate mixture collected in the trap was taken and its composition wasdetermined by measuring its refractive index and by comparing it with astandard graph. Additional feed mixture was provided as necessary to thefeed compartment.

From PV data, the membrane performance was studied by calculating thetotal flux, J_(p), and separation selectivity, α_(sep) as,$\begin{matrix}{J_{p} = \frac{W_{p}}{At}} & (3) \\{\alpha_{sep} = \frac{P_{w}/P_{{Iso} - {OH}}}{F_{w}/F_{{Iso} - {OH}}}} & (4)\end{matrix}$

In the above equations, W_(p) is mass of the permeate, A is the area ofthe membrane in contact with the feed mixture and t is time; P_(w) andP_(Iso-OH) are the mass % of water and isopropanol respectively, in thepermeate; F_(w) and F_(Iso-OH) are the mass % of water and isopropanolin the feed mixture, respectively; C_(w) ^(P) and C_(W) ^(F) areconcentrations of permeate and feed mixture, respectively. The resultsof pervaporation flux and α_(sep) are presented in Table III. TABLE IIIPervaporation Flux and Separation Selectivity at Different Mass % ofWater in the Feed Mixture at 30° C. for Different Membranes Mass % ofWa- ter in the Jp ×10² (kg/m²h) eq. (3) α_(sep) eq. (4) Feed NaAlgNaAlg-1 NaAlg-2 NaAlg NaAlg-1 NaAlg-2 10 5.0 — — 3591 — — 20 14.6 14.717.6 96.0 96.0 53.1 30 22.6 23.2 32.5 49.5 44.3 36.6 40 31.8 46.1 56.628.5 23.5 19.9 50 44.0 58.5 76.8 14.4 13.3 11.5 60 51.1 68.6 84.3 11.112.0 7.5 70 — 112.2 — — 10.9 — 80 88.4 — — 8.59 — —

The results of pervaporation flux and separation selectivity of all themembranes at 30° C. are presented in Table III. It is generally observedthat flux increases from neat NaAlg to NaAlg-1 and NaAlg-2 membranes;similarly an increase in flux is observed with an increasing amount ofwater in the feed mixture. On the contrary, separation selectivitydecreases from NaAlg to NaAlg-2 membrane as well as with increasingamount of water in the feed mixture. High permeation separation index isobserved for neat sodium alginate membrane.

Example 7 Diffusion Coefficients

The diffusion coefficient, Di of the solvent mixtures through membranematerials was calculated using the PV results as described inKusumocahyo, S. P.; Sudoh, M, J Membr Sci 1999, 161, 77 and Binning, R.C.; Lee, R. J.; Jennings, J. F.; Martin, E. C. Ind Eng Chem 1961, 53,45. $\begin{matrix}{J_{i} = {{P_{i}\left\lbrack {p_{i{({feed})}} - p_{i{({permeate})}}} \right\rbrack} = {\frac{D_{i}}{h}\left\lbrack {C_{i{({feed})}} - C_{i{({permeate})}}} \right\rbrack}}} & (5)\end{matrix}$

The D_(i) is assumed to be constant across the effective membranethickness, h; C_(i(feed)) and C_(i(permeate)) are respectively,composition of the liquids present in the feed and in the permeate. Thecomputed values of D_(i) (where the subscript i stands for water orisopropanol) at 30° C. are presented in Table IV. TABLE IV DiffusionCoefficients of Water and Isopropanol Calculated from Eq. (5) at 30° C.for Different Membranes Mass % of Wa- ter in the D_(w) ×10¹⁰ (m²/s) eq.(5) D_(ISO-OH) ×10¹⁰ (m²/s) eq. (5) Feed NaAlg NaAlg-1 NaAlg-2 NaAlgNaAlg-1 NaAlg-2 10 4.6 — — 0.01 — — 20 15.4 15.4 18.7 0.60 0.64 1.40 3027.5 28.3 39.7 1.29 1.49 2.54 40 45.8 66.8 82.7 2.41 4.26 6.22 50 78.7105 140 5.47 7.94 1.22 60 120 160 211 9.03 11.1 2.34 70 — 355 — 28.018.7 — 80 480 — — — — —

As expected, diffusion coefficients of water increase considerably withincreasing amount of water in the feed mixture suggesting that themembranes are water-selective. Such an increase in the amount of wateris dramatic at higher compositions of water in the feed mixture. Theincrease in D_(i) values with increasing amount of water in the feedmixture is attributed to the creation of extra free volume of themembrane matrix. Similarly, diffusion coefficients of isopropanol, eventhough they are quite smaller in magnitude than those observed forwater, show a slight increase with an increasing amount of water in thefeed mixture, except in the case of the NaAlg-2 membrane.

As regards the nature of the membranes, diffusion values show asystematic trend i.e., with increasing mass % of water in the feedmixture, the D_(i) values also increase systematically from NaAlg toNaAlg-2. In case of isopropanol, diffusion coefficients increase fromneat NaAlg to NaAlg-1, and to a lesser extent from NaAlg-1 to NaAlg-2membranes.

Example 8 Effect of Temperature

Temperature dependence of pervaporation flux and diffusivity results forthe feed mixture containing 20 mass % of water presented in Table V wereused to estimate the activation parameters in the Arrhenius equation:J _(p) =J _(P0) exp (−E _(P) /RT)   (6)

Here, E_(P) is activation energy for permeation, J_(P0) is permeationrate constant, R is gas constant and T is temperature in Kelvin. Ifactivation energy is positive, then permeation flux increases withincreasing temperature, which is generally observed in PV experiments.(See Burshe, M. C.; Netke, S. A.; Sawant, S. B.; Joshi, J. B.;Pangarkar, V. G. Sep Sci Technol 1997, 32, 1335; and Nam, S. Y.; Lee, Y.M. J Membr Sci 1999, 157, 63.) TABLE V Pervaporation Flux and SeparationSelectivity at Different Temperature for 20 Mass % of Water in the FeedMixture for Different Membranes Jp ×10² (kg/m²h) eq. (3) α_(sep), eq.(4) Temp. NaAlg- NaAlg- NaAlg- NaAlg- (° C.) NaAlg 1 2 NaAlg 1 2 30 13.815.1 17.2 96.0 96.0 53.1 40 19.1 18.1 19.7 68.7 60.0 43.1 50 25.9 20.521.9 55.3 53.1 30.8

The driving force for mass transport, which represents the concentrationgradient resulting from the difference in partial vapor pressure ofpermeants between the feed and the permeate, increases with increasingtemperature. As the feed temperature increases, the vapor pressure inthe feed compartment also increases, but the vapor pressure at thepermeate side is not affected resulting in increase of driving force athigher temperatures. The apparent activation energy data for permeation,E_(P) calculated from the slopes of the straight lines of the Arrheniusplots using the least squares method are presented in Table VI. TheE_(P) values vary according to the sequence: NaAlg >NaAlg-1> NaAlg-2.TABLE VI Permeation and Diffusion Activation Energies, Heat of Sorptionof Water and Energy Difference Values of the Membranes Parameters NaAlgNaAlg-1 NaAlg-2 E_(P) (kJ/mol) eq. (6) 5.60 1.59 .84 E_(D) (kJ/mol) eq.(7) 7.07 4.05 2.38 ΔH_(S) (kJ/mol) 1.49 2.47 2.54 E_(ISO-OH) − E_(w)(kJ/mol) 6.63 4.24 2.16

Similarly, results of mass transport due to activated diffusion aredescribed by the equation:D _(i) =D _(i0) exp(−E _(D) /RT)   (7)Here, E_(D) is energy of activation for diffusion and i represents wateror isopropanol. Arrhenius plots are linear in the temperature intervalstudied. Heat of sorption values have been calculated as:ΔH_(S)(≅E_(P)−E_(D)). These results are included in Table V. The ΔH_(S)values are negative in all the cases suggesting an endothermic processfor sorption. The E_(D) values of the membranes show the sequence:NaAlg>NaAlg-1>NaAlg-2.

The temperature dependency of α_(sep) was investigated using therelationship described in Ping, Z. H.; Nguyen, Q. T.; Clement, R.; Neel,J. J Membr Sci 1990, 48, 297, which is as follows: $\begin{matrix}{Y_{w} = \frac{1}{1 + {\left( {J_{{Iso} - {OH}}/J_{w}} \right){\exp\left( {{- \left( {E_{{Iso} - {OH}} + E_{w}} \right)}/{RT}} \right)}}}} & (8)\end{matrix}$where Y_(W) is water composition in the permeate, J_(W) and J_(Iso-OH)are permeation fluxes; E_(W) and E_(Iso-OH) are Arrhenius activationenergies for water and isopropanol, respectively at the average energylevel. A positive value of [E_(Iso-OH)−E_(W)] indicates that α_(sep)decreases with increasing temperature, while negative value indicatesthat α_(sep) increases with an increase in temperature. For all themembranes, the difference (E_(Iso-OH)−E_(W)) is positive (see Table V)further supporting that α_(sep) decreases with increasing temperature.

Example 9 Synthesis of Glutaraldehyde Crosslinked Membranes

The following membranes were used in Examples 10-13. The mediumviscosity grade sodium alginate (viscosity of 230 mPa.s for 5% solution)was a reagent grade chemical in the present example. High viscositygrade sodium alginate (viscosity of 170 mpa.s for 1% solution), lowviscosity grade sodium alginate (with a viscosity of 138 mPa.s for 5%solution, all viscosities measured at the shear rate of 69.8 l/s, usingthe Brookfield Rheometer, Model DV-III at 30° C.), PVA acetic acid,methanol, glutaraldehyde (25% in water) and PEG (200) were all of ARgrade samples. Double distilled water was used throughout this example.

To fabricate a pure sodium alginate membrane, 5 g of polymer wasdissolved in 100 mL of distilled water. The solution was filteredthrough a cotton plug, cast on a glass plate and evaporated to drynessin a dust free atmosphere at room temperature. The membrane formed waspeeled off from the glass plate and crosslinked by immersion in anacidic solution of methanol:water (75:25 by volume) and also containing1 vol % of glutaraldehyde. After immersion for 12 h the membrane wasdried again at room temperature. The membranes of the present exampleprepared with low, medium and high viscosity grade SA polymers aredesignated, respectively as SA-LV, SA-MV and SA-HV. Such membranes wereprepared as above by solution casting method. Due to high water uptakecapacity, the high viscosity grade SA polymer required more of water tosolubilize and also it was difficult to remove the undissolved particlesby filtration. In addition, it flows out of the casting glass plateposing further difficulty in controlling membrane thickness. Due todifferences in solution viscosity and increased water retainingcapacity, the drying rate of the membranes with low and medium viscositywas much less when compared to the membranes prepared from highviscosity polymer.

Based on membrane performance, SA-LV sample was selected for furthermodification. Different variations of this membrane were prepared byadding 10 mass % of PEG and varying amounts of PVA from 5 to 20 mass %.During formation a previously prepared PVA and PEG solution was added tothe SA solution and stirred for 2 h at room temperature. The membranesthus prepared are designated as SA-1, SA-2 and SA-3, and respectivelycontained 5, 10 and 20 mass % of PVA. The membranes were crosslinkedwith glutaraldehyde in an acidic solution of water:ethanol in the volumeratio of 75:25. Ethanol, being a nonsolvent, helped to prevent theinitial dissolution of the membrane and water present in the solutionresulted in membrane swelling, thus facilitating an easy penetration ofglutaraldehyde into the membrane matrix. The membranes were stored atroom temperature in a desiccator before they were used in PVexperiments.

The cross-linking reaction took place between —OH group of sodiumalginate and —CHO group of glutaraldehyde with the formation of etherlinkage by eliminating water, which is commonly observed in case ofcellulose-based hydrophilic polymers. It has been well documented in theliterature that SA membranes are rigid and have a broad free volumedistribution, so that polymer chains relax to achieve a narrow freevolume distribution, thus altering the PV performance of the membrane.Hence, to avoid these problems and to increase membrane flexibility aswell as membrane performance, PEG and PVA were added. Addition of 10mass % PEG was used in all membranes to produce a more flexiblemembrane. Other low mass % PEG amounts may be used to obtain a desiredbalance between flexibility and selectivity and flux.

In the same manner, PVA was added to increase the permeation flux.Although only 5, 10 and 20 mass % PVA was added to the membranes of thepresent example, it will be understood that other low mass % PVA amountsmay be used to obtain a desired flux in a given membrane. Specifically,any mass % between 5 and 20 should be acceptable.

Example 10 Equilibrium Sorption and Sorption Selectivity (α_(sorp))

Equilibrium sorption was studied in the same way as explained in Example5. These data are presented in Table VII. TABLE VII Equilibrium Mass %Uptake (S) of the Membranes at 30° C. Mass % of Water in the S (kg/kg)%, Feed SA-LV SA-MV SA-HV SA-1 SA-2 SA-3 10 19.90 28.06 25.47 31.1041.21 55.37 20 27.72 31.18 32.51 40.33 52.01 63.64 30 38.84 41.45 44.2549.94 61.47 72.99 40 48.41 48.09 58.20 63.51 67.34 76.29 50 56.38 59.8391.23 68.78 78.51 82.41 60 61.17 67.33 124.32 77.20 85.20 82.78 70 63.4170.99 152.63 80.24 84.32 86.40

Sorption selectivity was calculated in the same manner as was done inExample 5. The cold trap surrounded by liquid nitrogen was heated to120° C. (close to the boiling temperature of HAc, 117.5° C.). The vaporwas condensed in a cold trap surrounded by a liquid nitrogen jar.Composition of the condensed liquid mixture was then calculated bymeasuring the refractive index (accurate up to ±0.0002 units). Sorptionselectivity was calculated as before using Eq. 2.

PV tests were performed in the same manner as explained in Example 6.The mass % water in acetic acid mixture was varied from 10 to 50. Aftertaking 150 mL of the mixture in the feed compartment, the test membranewas allowed to equilibrate for 2 h. The downstream vacuum pressure was 0torr.

The results of % equilibrium mass uptake (S) of the neat sodium alginatemembranes (SA-LV, SA-MV and SA-HV) as well as the modified sodiumalginate membranes (SA-1, SA-2 and SA-3) from low viscosity grade sampleat 30° C. are presented in Table VII. In case of membranes prepared fromlow viscosity and medium viscosity grade sodium alginate samples, mass %equilibrium values increase systematically with increasing amount ofwater in the feed mixture.

In case of membranes prepared from high viscosity grade sodium alginatesamples, the % equilibrium mass uptake values increase steadily up to 40mass % of water in the feed mixture and later it increases considerablyresulting in equilibrium swelling up to 150 mass %. The % equilibriumsorption results of the modified membranes (SA-1, SA-2 and SA-3)prepared from low viscosity SA grade polymer, PEG and PVA are higherthan those of the neat sodium alginate membranes.

The % composition of water in each membrane was calculated as a functionof mass % of water in the feed mixture and these data are presented at30° C. in Table VIII for neat as well as modified sodium alginatemembranes. It was observed that there is not much variation in %composition of water for SA-LV, SA-MV and SA-HV membranes, but generallyan increase in % composition of water in the membrane was observed withthe increasing amount of water in the feed mixture. The same trend isalso observed with the modified membranes, SA-1, SA-2 and SA-3.

The results of sorption selectivity of the membranes at 30° C. for theneat sodium alginate and the modified SA-LV membranes are also presentedin Table VIII. In both cases, sorption selectivity decreasesconsiderably with increasing amount of water (say up to about 50 mass %)in the feed mixture and then levels off. Sorption selectivity variesonly slightly with the type of sodium alginate in the membrane. Sorptionselectivity generally decreases with increasing viscosity of the sodiumalginate samples. Similarly, with increasing amount of PVA in themodified membranes, sorption selectivity decreases systematically overthe entire composition range of the feed mixture. TABLE VIII %Composition of Water Sorbed in the Membranes and Sorption Selectivity(α_(sorp)) at 30° C. Mass % of Water in the Feed SA-LV SA-MV SA-HV SA-1SA-2 SA-3 % Composition of Water in the Membrane 10 61.50 61.00 61.0057.00 52.50 47.00 20 70.00 72.50 68.00 63.00 57.00 51.25 30 71.00 69.7560.00 65.25 61.50 60.50 40 71.75 69.50 69.25 68.00 66.00 65.50 50 74.0072.00 71.50 72.50 70.00 72.50 60 76.00 77.00 73.50 76.50 75.50 74.00 7082.50 76.00 79.50 82.25 82.00 79.00 Sorption Selectivity, α_(sorp), 1014.2 14.1 13.5 10.92 9.80 7.85 20 9.24 10.4 8.47 6.78 5.28 4.20 30 5.715.36 3.48 4.41 3.62 3.52 40 3.79 3.39 3.74 3.15 2.90 2.84 50 2.85 2.552.51 2.72 2.33 2.64 60 2.11 2.20 1.86 2.18 2.06 1.92 70 2.03 1.35 1.682.00 1.97 1.62

Example 11 Membrane Performance

Membranes performance was studied by calculating total permeation flux,J_(p) and separation selectivity, α_(sep) as in the previous examples.The results of pervaporation flux and separation selectivity arepresented in Table IX for the neat sodium alginate as well as themodified SA-LV membranes. For all the membranes, the pervaporation fluxincreases considerably with an increase in mass % of water in the feedmixture up to 40 mass % of water and then levels off. This may be theresult of the interaction between water and sodium alginate resulting inan increased swelling of the membrane due to facilitated diffusion ofthe permeant molecules.

The increase in flux remains constant beyond 40 mass % of water in thefeed mixture for all the membranes. Even though the difference in fluxof the membranes is negligible at higher water content of the feedmixture, but they vary slightly at lower water content signifying theeffect of polymer viscosity at lower water content of the feed mixture.Chiang and Hu²⁶ observed similar effects in their study on PV separationof water-ethanol mixtures using the blend membranes of PVA-g-MMA/MA. Theflux of modified SA-LV membranes increases with an increasing amount ofwater in the feed mixtures. The modified membrane (SA-3) with higheramount of PVA showed higher flux than all the other membranes.

The results of % composition of water in the permeate mixture at 30° C.are included in Table IX. Generally, for the neat sodium alginatemembranes, the amount of water in the permeate mixture decreasesslightly with increasing viscosity of the SA polymer used. On thecontrary, a considerable decrease in mass % composition of water isobserved in permeate mixture with increasing amount of PVA in themodified membranes. This increase in water content of the neat sodiumalginate membranes may be due to the increased selective interaction ofthe permeate water molecules with —COOH and —OH groups of sodiumalginate while the decrease in water content in low viscosity grademodified membranes may be due to higher plasticization effect of waterresulting in an increased chain mobility and increased free volume ofthe polymer, thereby accommodating more of acetic acid molecules in themembrane matrix.

The results of separation selectivity presented in Table IX show theleast effect with polymer viscosity, but the values of α_(sep) decreasewith increasing amount of water in the feed mixture. On the other hand,α_(sep) vary drastically with the polymer modifications i.e., thesevalues decrease considerably with an increasing amount of PVA in themembrane. In case of neat SA membranes (SA-LV, SA-MV and SA-HV)separation selectivity increases considerably at 20 mass % of water andthereafter decreases rapidly up to 30 mass % of water and then remainsconstant beyond 40 mass % of water in the feed mixture. With themodified membranes, separation selectivity follows a systematicdecreasing trend up to 20 mass %, beyond which it remains constant.TABLE IX Pervaporation Flux, % Composition of Water in Permeate andSeparation Selectivity at Different Mass % of Water in the Feed Mixtureat 30° C. for Different Membranes Mass % Water in Feed SA-LV SA-MV SA-HVSA-1 SA-2 SA-3 Jp ×10² (kg/m² · h), 10 2.20 3.69 3.72 2.39 4.25 7.39 206.90 14.5 14.5 16.3 24.8 25.6 30 25.8 30.2 27.4 29.1 32.7 40.8 40 48.547.3 48.5 47.1 49.9 53.3 50 47.7 47.7 47.7 52.3 58.1 76.0 % Compositionof Water in the Permeate 10 63.5 62.0 61.5 81.8 70.0 54.0 20 84.5 81.579.5 72.0 72.0 67.0 30 73.0 73.0 72.3 69.5 67.0 61.5 40 72.5 72.5 78.874.5 72.3 70.0 50 73.0 78.0 80.0 79.5 77.8 80.0 Separation Selectivity,α_(sep), 10 15.7 14.7 14.4 40.3 21.0 10.6 20 21.8 17.6 15.5 10.3 10.38.12 30 6.31 6.31 6.08 5.32 4.74 3.73 40 3.96 3.96 5.56 4.38 3.91 3.5050 2.70 3.55 4.00 3.88 3.50 4.00

Example 12 Diffusive Transport

The computed values of water (D_(w)) and acetic acid (D_(HAc)) at 30° C.are presented in Table X. Diffusion of water is higher than HAc in allthe membranes studied. The diffusion coefficients increase considerablywith increasing amount of water in the feed mixture suggesting waterselectivity of the membranes. Such as increase in D_(w) with increasingamounts of water in the feed mixture is attributed to the creation ofextra free volume in the membrane matrix thus facilitating the watertransport through the pore volume. In addition, D_(i) increasedsystematically with an increase in the amount of PVA in the modifiedmembranes. However, diffusion trends of both water and acetic acid forthe neat SA as well as the modified SA membranes are not very dependentupon the viscosity or even the modifications of the polymers. TABLE XDiffusion Coefficients of Water and Acetic Acid Calculated from eq. (5)at 30° C. for Different Membranes Mass % of Water in the Feed SA-LVSA-MV SA-HV SA-1 SA-2 SA-3 D_(w) ×10¹⁰ (m²/s), 10 2.17 3.66 3.70 2.274.13 7.56 20 7.55 16.0 16.2 28.6 18.8 30.4 30 36.4 42.7 39.1 42.7 49.366.3 40 90.2 87.9 82.1 84.8 93.1 104 50 126 111 106 118 136 169 D_(HAc)×10¹⁰(m²/s), eq. (5) 10 1.24 2.24 2.32 0.51 1.77 6.44 20 1.38 36.2 4.1611.1 7.31 15.0 30 13.5 15.8 15.0 18.7 24.3 41.5 40 34.2 33.3 22.1 29.035.7 44.4 50 46.7 31.2 26.5 30.3 38.7 42.2

Example 13 Effect of Temperature

For temperature variation studies, only 20 mass % water containingmixture was employed during membrane formation and the pervaporationflux and separation selectivity of these membranes were studied at 30,40 and 50° C. These data for all the membranes are summarized in TableXI. In all cases, pervaporation flux increased with increasingtemperature, but a reverse trend is observed for separation selectivity.The results of permeation flux have been derived by the Arrheniusequation. If E_(p) is positive, then permeation flux increases withincreasing temperature as observed in the prior art. Driving force formass transport also increases with increasing temperature. The values ofE_(p) calculated from the slopes of the straight lines of the Arrheniusplots by the least squares method are presented in Table XII. The E_(p)values do not show any systematic trend with the type of SA or modifiedSA used.

The temperature dependent mass transport due to activated diffusion wasalso derived by the Arrhenius equation. The Arrhenius plots of log D_(i)vs 1000/T for water diffusion are also linear in the temperatureinterval studied and the E_(D) values are positive in all the cases;these do not show any systematic variation with the nature of themembrane. The heat of sorption values ΔH_(S) calculated from thedifference: (≅E_(p)−E_(D)) included in Table VII are negative in allcases suggesting an endothermic sorption process.

The temperature dependency of α_(sep) was also estimated as describedabove. A positive value of [E_(HAc)−E_(w)] indicates that α_(sep)decreases with increasing temperature, but a negative value indicatesthat α_(sep) increases with an increase in temperature. For all themembranes, the difference is positive (see Table XII) further supportingthe observation of decreases in α_(sep) with increasing temperature.TABLE XI Pervaporation Flux, Separation Selectivity and DiffusionCoefficients of Water and Acetic Acid at Different Temperatures for 20Mass % of Water in the Feed Mixture Temp. ° C. SA-LV SA-MV SA-HV SA-1SA-2 SA-3 Jp ×10² (kg/m²h), 30 6.91 14.5 14.5 11.2 13.3 22.7 40 14.219.7 17.7 18.2 21.7 26.0 50 15.8 26.3 24.2 26.2 31.2 33.5 SeparationSelectivity, α_(sep), eq. (4) 30 21.8 17.6 15.5 11.2 10.3 5.88 40 16.014.6 10.6 8.90 9.22 5.09 50 12.0 11.7 9.33 7.27 8.12 4.33 D_(w) ×10¹⁰(m²/s), eq. (5) 30 7.54 16.0 16.2 12.8 15.3 28.5 40 15.7 22.1 20.4 21.325.3 33.7 50 18.0 29.9 28.3 31.7 37.1 45.3 D_(HAc) ×10¹⁰ (m²/s), eq. (5)30 1.4 3.6 4.16 4.6 5.96 19.4 40 3.9 6.0 7.72 9.6 11.0 26.5 50 6.0 10.312.1 17.4 18.3 41.8

TABLE XII Permeation and Diffusion Activation Energies, Heat of Sorptionof Water and Energy Difference Values of the Membranes ActivationParameters SA-LV SA-MV SA-HV SA-1 SA-2 SA-3 E_(P) (kJ/mol) 29.19 47.2018.49 29.31 31.89 10.31 eq.(6) E_(D) (kJ/mol) 35.60 52.30 25.63 37.0035.85 18.88 eq.(7) ΔH_(S)(kJ/mol) −6.41 −5.10 −7.14 −7.69 −3.96 −8.57E_(HAc) − E_(W) 24.40 92.43 17.88 17.74 9.50 12.38 (kJ/mol)

Example 14 Synthesis of Polyacrylenitrile-g-Polyvinyl Alcohol Membranes

The following membranes were used in Examples 15-18. PVA (mol. wt.1,25,000), analytical grade acrylonitrile (AN), laboratory reagent gradeglutaraldehyde (25% content in water), analytical reagent grade samplesof dimethyl formamide (DMF), and reagent grade ceric ammonium nitrate(CAN), dimethyl sulfoxide (DMSO), hydrochloric acid and acetone wereused in the following examples. All the chemicals were used withoutfurther purification. Double distilled deionized water was usedthroughout the examples.

In a three-necked round bottom flask fitted with a condenser, gas inletand a thermometer, about 10 g of PVA was dissolved in 100 mL of DMSO at60° C. under constant stirring in a nitrogen atmosphere. After coolingthe solution, AN was added by stirring. To this, 5 mL of 0.1M of cerricammonium nitrate (CAN) was added and the reaction mixture was maintainedbetween 50 and 60° C. for 4 h. The polymer was precipitated by addingexcess acetone, filtered through a suction pump and dried in a vacuumoven at 60° C. Two copolymers with % grafting of 46 and 93 (designated,respectively as PVA-1 and PVA-2) were prepared by using 5 and 10 g ofAN, respectively. In both membranes a 100% grafting efficiency wasachieved with 92% conversion of AN.

Saturated amounts of neat PVA, and PVA-1 and PVA-2 polymers weredissolved separately in 100 mL of DMSO at 50° C. with constant stirringunder a slow stream of nitrogen gas and the solution was cooled to roomtemperature. To these polymer solutions, 0.0035 mol of glutaraldehydeand 0.5 mL of 1N HCl were added and stirred for 30 min to achieve aneffective cross-linking of the copolymer. Films were cast on clean glassplates by pouring uniformly the polymer solutions under controlledhumidity conditions. Membranes were dried at room temperature in a dustfree atmosphere and cured at 60° C. in an oven. The dried membranes werepeeled off from the glass plates and washed with water repeatedly toremove the excess glutaraldehyde and HCl, and then allowed to dry atroom temperature for 24 h.

Example 15 Polymer Characterization

Prior to membrane formation, the copolymers were characterized by FTIRspectra scanned in the range 4000-400 cm⁻¹ using KBr pellets on aNicolet spectrometer (Model, Impact 410, USA). A broad band appearing at˜3382 cm⁻¹ corresponds to —OH stretching vibrations of the hydroxylgroups of PVA; the peak at 2938 cm⁻¹ is assigned to aliphatic —C—Hstretching vibration; the peak at 1647 cm⁻¹ corresponds to carbonylgroup. A strong band appearing at ˜2246 cm⁻¹ corresponds to —CNstretching vibrations of acrylonitrile thus confirming grafting reactionbetween PAN and PVA.

Example 16 Swelling Experiments

Swelling experiments were performed in water and DMF mixtures ofdifferent compositions at 25±0.5° C. in an electronically controlledoven (WTB Binder, Model, BD-53, Germany) as per the procedures known inthe art. Polymer samples were placed inside the air-tight test bottlescontaining different mixtures of water and DMF. Test bottles were placedin the oven maintained at 25° C. After 24 h (i.e., after completeattainment of equilibrium), membranes were removed and thesurface-adhered solvent drops were removed using soft filter papers andweighed immediately. The degree swelling (DS) was calculated by takingthe ratio of equilibrium mass, W_(∞) to that of dry mass W_(o) of themembrane by using, $\begin{matrix}{{DS} = {\frac{W_{\infty}}{W_{0}}.}} & (9)\end{matrix}$

Example 17 Pervaporation Tests

Pervaporation experiments were carried out for water-DMF mixtures usingthe apparatus described above for sodium alginate membranes. Thecomposition of DMF was varied from 10 to 90 mass % at 25° C. From the PVdata, separation selectivity, α_(sep) and permeation flux werecalculated similarly as described for the sodium alginate membranesabove. The results of pervaporation flux and separation selectivity fordifferent mass % of water in the feed mixture at 25° C. are presented inTable XIII. TABLE XIII Total Pervaporation Flux and SeparationSelectivity Data for Different Mass % of Water in the Feed Mixture at25° C. Mass % Jp 10² (kg/m²h) α_(sep) water PVA PVA-1 PVA-2 PVA PVA-1PVA-2 10 1.6 0.9 0.18 17.1 18.1 21.2 20 5.0 4.2 1.0 22.7 20.3 24.0 306.5 3.3 2.2 26.8 27.1 29.4 40 7.4 5.5 1.8 28.5 30.0 31.1 50 9.2 9.7 5.324.0 28.8 34.1 60 11.1 9.1 7.5 26.0 32.1 33.4 70 14.0 12.3 7.3 18.2 30.136.7 80 15.5 12.3 10.8 16.4 24.5 31.2 90 20.0 16.4 9.3 11.0 15.0 23.9

Example 18 Membrane Performance

Degree of swelling increases with an increase in mass % of water in thefeed for all the membranes. Degree of swelling is higher for neat PVAwhen compared to PVA-1 and PVA-2 membranes, the lowest value is observedfor PVA-2. Thus, degree of swelling decreases with an increase in %grafting of the copolymer.

The total permeation flux for PVA is higher than those of the graftedmembranes (PVA-1 and PVA-2). Decreasing trend in the J_(p) values withincreasing grafting of the copolymer membranes may be the result ofincreased hydrophobicity of the copolymer resulting in lower water flux.As the mass % of water in the feed mixture increases, hydrophilicinteractions also increase and hence, the total flux values increasesystematically with increasing water content in the feed mixture (seeTable XIII).

The results of α_(sep) also included in Table XIII show a steadyincrease from PVA to PVA-2 membrane i.e., with increasing grafting ofthe membranes. The α_(sep) values of all the membranes are maximumbetween 40 and 70 mass % of water in the feed mixture and beyond thismass % water they show a decreasing trend. It is observed that α_(sep)is optimum at 40 mass % of water for PVA, while for PVA-1, the optimumvalue of α_(sep) is observed at 60 mass % of water in the feed. However,PVA-2 membrane exhibits the maximum α_(sep) of 36.7 at 70 mass % ofwater in the feed mixture. Thus, separation selectivity of the membranesdepends on polymer morphology.

Pervaporation flux of water and DMF as a function of mass % of water inthe feed are presented at 25° C. in Table XIV. For PVA membrane, thewater flux (J_(w)) increases systematically with increasing amount ofwater in the feed mixture, but for PVA-1 and PVA-2 membranes, the fluxdoes not vary systematically with the amount of water present in thefeed mixture. In general, there is an increase of water flux withincreasing amount of water in the feed mixture. Similarly, the flux dataof DMF for PVA, PVA-1 and PVA-2 membranes, even though decreasegenerally with increasing amount of water in the feed mixture, but theeffect is not very systematic. A comparison of flux data of water andDMF in Table XIV indicate that water has higher flux than DMF;particularly more so at higher amounts of water in the feed mixture.TABLE XIV Pervaporation Flux Data of Water and DMF at Different Mass %of Water in the Feed Mixture at 25° C. for Different Membranes Mass %J_(W) 10² (kg/m²h) J_(DMF) 10² (kg/m²h) water PVA PVA-1 PVA-2 PVA PVA-1PVA-2 10 1.1 0.6 0.13 0.55 0.30 0.05 20 4.3 3.5 0.86 0.75 0.69 0.14 306.0 3.1 2.0 0.52 0.26 0.16 40 7.0 5.2 1.7 0.37 0.26 0.08 50 8.8 9.4 5.20.37 0.33 0.15 60 10.8 8.9 7.4 0.28 0.19 0.15 70 13.7 12.1 7.2 0.32 0.170.08 80 15.3 12.2 10.7 0.23 0.12 0.09 90 19.8 16.3 9.3 0.20 0.12 0.04

The effect of temperature on pervaporation flux and selectivity wasinvestigated for the feed mixture containing 10 mass % of water. Thesedata at 25, 35 and 45° C. are presented in Table XV. With increasingtemperature, flux also increases while selectivity decreases. Fluxdecreases from neat PVA to PVA-1 and PVA-2 at all the temperatures, thusshowing the effect of extent of polymer grafting. On the other hand,α_(sep) values increase from neat PVA to PVA-1 and PVA-2 membranes atall the temperatures. These results are in conformity with the findingsof Neel et al., who considered that selective diffusion in the dryregion of the membrane at the downstream side is important to determinethe overall membrane selectivity. Similarly, Mulder et al. interpretedthe selectivity data as due to the preferential sorption of one of thecomponents of the binary mixture with the swollen polymeric membrane atthe upstream side. The present membranes are preferentially moreselective to water than DMF. TABLE XV Pervaporation Flux and SeparationSelectivity at Different Temperatures for 10 Mass % of Water in the FeedMixture Temp Jp 10² (kg/m²h) α_(sep) (° C.) PVA PVA-1 PVA-2 PVA PVA-1PVA-2 25 1.6 0.9 0.18 17.1 18.12 21.2 35 3.6 1.5 1.2 13.2 15.41 19.13 4512.6 5.3 4.01 6.5 13.52 16.41

The diffusion coefficients, D_(i) of the permeants were calculated asbefore. Computed values of D_(i) (where the subscript i stands for wateror DMF) at 25° C. are presented in Table XVI. Values of D_(W) for waterare much higher than those observed for DMF in all the membranes at allthe feed compositions of water, further suggesting that membranes ofthis study are more water selective than DMF. This is further supportedby the fact that with an increasing amount of water in the feed mixture,the D_(w) values also increase considerably, but not so much for DMF.Since PVA is a more hydrophilic polymer than the grafted copolymermembranes (PVA-1 and PVA-2), diffusion coefficients of water and DMF arehigher for PVA than those observed for the grafted copolymer membranes(PVA-1 and PVA-2). Among the grafted copolymer membranes, PVA-1 with alower grafting ratio has higher diffusivity than PVA-2 membrane.Diffusion coefficients for 10 mass % of water in the feed mixture atdifferent temperatures are presented in Table XVII. Values of D_(i) forboth water and DMF increase with an increase in temperature. Also,diffusion of water is always higher than that of DMF for all themembranes and at all the temperatures. TABLE XVI Diffusion Coefficientsof Water and DMF Calculated from Eq. 5 at 25° C. Mass % D_(W) ×10⁹(m²/s) D_(DMF) × 10⁹ (m²/s) water PVA PVA-1 PVA-2 PVA PVA-1 PVA-2 102.64 1.48 0.29 1.39 0.74 0.12 20 9.15 7.73 1.83 1.61 1.52 0.30 30 13.506.93 4.56 1.18 0.60 0.36 40 17.90 13.23 4.34 0.94 0.66 0.21 50 26.8828.22 15.29 1.12 0.68 0.45 60 40.40 32.93 27.06 1.04 0.68 0.54 70 69.1359.37 34.88 1.63 0.85 0.41 80 115.6 89.76 78.10 1.76 0.92 0.63 90 308.0246.0 135.9 3.11 1.82 0.63

TABLE XVII Diffusion Coefficients of Water and DMF Calculated from Eq. 4at Different Temperatures for 10 Mass % of Water in the Feed MixtureTemp D_(W) × 10⁹(m²/s) D_(DMF) × 10⁹ (m²/s) (° C.) PVA PVA-1 PVA-2 PVAPVA-1 PVA-2 25 2.64 1.48 0.29 1.39 0.74 0.12 35 6.06 2.50 1.97 4.12 1.469.27 45 23.15 8.90 6.64 3.20 5.93 3.64

Temperature-dependent results of permeation flux (Table XV) anddiffusion (Table XVI) have been analyzed by the Arrhenius equations asexplained before. The E_(P) and E_(D) values estimated by the method ofleast squares are given in Table XVIII. By using the E_(P) and E_(D)values for water, one may further compute the heat of sorption, ΔH_(S)for water permeation using the empirical relation: ΔH_(S)=E_(P)−E_(D)and these data are also included in Table XVIII. The ΔH_(S) values arenegative in all the cases suggesting an endothermic sorption mode. Thetemperature dependence of separation selectivity (Table XV) wasanalyzed. A positive value of (E_(DMF)−E_(W)) indicates that α_(sep)decreases with increasing temperature and negative values indicate thatα_(sep) increases with an increase in temperature. In the presentinvestigation, the values of (E_(DMF)−E_(W)) are positive, indicatingthat α_(sep) decreases with increasing temperature as shown in Table XV.TABLE XVIII Permeation and Diffusion Activation Energies, Heat ofSorption for Water and Energy Difference Values for Different MembranesParameter PVA PVA-1 PVA-2 E_(P) (kJ/mol), 63.73 65.34 119.3 Eq. (6)E_(D) (kJ/mol), 85.30 160.1 123.2 Eq. (7) ΔH_(S) (kJ/mol) −21.57 −94.78−3.48 E_(DMF) − E_(W) 37.83 11.55 10.10 (kJ/mol)

Example 18 Hybrid Purification System

A novel hybrid process may be used wherein one can combine with otherpurification or recovery steps a pervaporation process to remove theorganics from water at the azeotropic concentration of water where theprocess is very economic compared to distillation or adsorption.Pervaporation may be combined with the electrodialysis process to removethe unwanted salts from the hard water.

1. An acrylamide grafted alginate membrane comprising: an acrylamidemonomer and an alginate polymer in a ratio of between about 1:2 and 1:1;and a glutaraldehyde cross-linker.
 2. A membrane according to claim 1,wherein the alginate comprises sodium alginate.
 3. A membrane accordingto claim 1, wherein the membrane further comprises polyethylene glycol(PEG).
 4. A membrane according to claim 3, wherein the membrane furthercomprises approximately 10 mass % PEG.
 5. A membrane according to claim1, wherein the membrane further comprises polyvinyl alcohol (PVA).
 6. Amembrane according to claim 5, wherein the membrane further comprisesapproximately 5 mass % PVA.
 7. A membrane according to claim 1, whereinthe membrane further comprises PEG and PVA.
 8. A membrane according toclaim 1, wherein the membrane is optimized for the separation of alcoholfrom water.
 9. A membrane according to claim 8, wherein alcohol isselected from the group consisting of isopropanol, methanol, ethanol,and any combinations thereof.
 10. A pervaporation device comprising: atleast two chambers; and a membrane affixed between the two chambers inmanner to prevent fluid flow from one chamber to the other; wherein themembrane comprises a poly(acrylamide)-grafted sodium alginate copolymerpervaporation membrane cross-linked with glutaraldehyde.
 11. A deviceaccording to claim 10, wherein the alginate comprises sodium alginate.12. A device according to claim 10, wherein the membrane furthercomprises polyethylene glycol (PEG).
 13. A device according to claim 1,wherein the membrane further comprises polyvinyl alcohol (PVA).
 14. Adevice according to claim 1, wherein the membrane is optimized for theseparation of alcohol from water.
 15. A device according to claim 14,wherein alcohol is selected from the group consisting of isopropanol,methanol, ethanol, and any combinations thereof.
 16. A method ofseparating alcohol from water comprising: placing a pervaporationmembrane between two chambers in a pervaporation device having at leasttwo chambers, wherein the membrane includes: an acrylamide monomer andan alginate polymer in a ratio of between about 1:2 and 1:1; and aglutaraldehyde cross-linker; supplying water comprising the alcohol toone chamber; and decreasing pressure in the second chamber on the otherside of the pervaporation membrane so that alcohol passes from thewater, through the pervaporation membrane, and into the second chamber.17. A method according to claim 16, wherein the alginate comprisessodium alginate.
 18. A method according to claim 16, wherein themembrane further comprises polyethylene glycol (PEG).
 19. A methodaccording to claim 16, wherein the membrane further comprises polyvinylalcohol (PVA).
 20. A method according to claim 14, wherein alcohol isselected from the group consisting of isopropanol, methanol, ethanol,and any combinations thereof.